Displays a warning when forcing CDAT_BUILD_MODE

Found this commit from some time ago.

If `CDAT_BUILD_ESGF` is enabled, then `CDAT_BUILD_MODE` gets forced to `LEAN`. This commit displays a warning to the user when that happens.
It can be set from the cache or the UI.

If the user selects CDAT_BUILD_MODE=DEFAULT and CDAT_BUILD_ESGF=on, then he gets the warning.